Every vessel is required to carry some kind of efficient sound producing device to signal their intentions as outlined below.
Vessels are required to sound signals any time that they are in close quarters and risk of collision exists.
The following signals are the only ones to be used to signal a vessel's intentions (inland rules only).
Note: Inland rules use sound signals to indicate intent to maneuver and a response should be received. In international rules the signals are given when the maneuver is being executed.
Vessels indicate their intention to maneuver by using sound signals. If you do not agree with or understand clearly what the other vessel's intentions are, you should sound the danger or doubt signal (5 short, rapid blasts). Each vessel should then slow or stop until signals for safe passing are sounded, understood and agreed to.
The danger or doubt signal can also be used to tell another vessel that its action is dangerous. If a boat is backing up into an obstruction you would sound the danger signal to warn the operator.